Semiconductor lasers are the subject of a well developed technology and have been incorporated into many commercial products. Further, there has been much work in the development of one-dimensional arrays of semiconductor lasers integrated into a single integrated circuit chip. These one-dimensional arrays are arranged such that the resonant cavities of the lasers extend horizontally in parallel along the surface of an integrated circuit chip. In the usual embodiments, the laser light is emitted from a side facet of the IC chip. However, there are known embodiments in which the light outputs of the horizontal cavities are reflected or Bragg diffracted to become perpendicular to the surface of the chip, that is, to be surface-emitting. It is possible to have two-dimensional arrays of Bragg diffracted or reflected, horizontal-cavity lasers, as has been disclosed by Yang et al in a technical article entitled "Monolithic two dimensional surface emitting arrays of GaAs/AlGaAs lasers" appearing in Proceedings of SPIE, volume 893, 1988, at pages 181-187. This article discusses a variety of two-dimensional arrays of surface-emitting semiconductor lasers.
There has been considerable effort in developing vertical-cavity, surface-emitting lasers, as has been discussed by Iga et al in a technical article entitled "Surface emitting semiconductor laser array: Its advantage and future" appearing in Journal of Vacuum Science Technology A, volume 7, 1989, at pages 842-846. The laser diodes discussed therein suffer the disadvantages of requiring regrowth for current confinement and etching through of the substrate in order to deposit a mirror on the bottom of the epitaxially grown cavity structure.
Jewell et al have disclosed a two-dimensional array of vertical-cavity, surface-emitting diode lasers in U.S. Pat. No. 4,949,350 incorporated herein by reference. These arrays have been further reported by Jewell et al in a technical article entitled "Low-threshold electrically pumped vertical-cavity surface-emitting microlasers" appearing in Electronics Letters, volume 25, 1989 at pages 1123-1124. The Jewell et al laser arrays required neither etch-through nor regrowth. Jewell et al grew on a substrate a horizontally uniform, vertical-cavity laser structure. The lasing active region was located between epitaxially grown semiconductor end mirrors, separated by the lasing wavelength. The mirrors were stacks of quarter-wavelength thick layers of semiconductor materials of differing refractive indices, thereby acting as Bragg reflectors. The uniform laser structure was further defined into an array of lasers by ion-beam assisted chemical etching which removed horizontal portions of the laser structure to leave an array of pillars rising approximately 5 .mu.m above the substrate. Each pillar, which was a separate laser, had a diameter of a few micrometers or more and the pillars were separated by similar distances. The conducting substrate served as one common electrode and the tops of the individual pillar lasers were coated with a metallic layer which served as a selectable electrode for the individual laser among many lasers in the array. Current flowed between the two electrodes vertically through the mirrors, the cavity and the active layer. The substrate, although electrically conducting, was transparent to the laser light so that the light was emitted through the planar bottom surface of the chip, that is, the lasers were surface-emitting lasers.
In the original experiment work of Jewell et al, the electrical contact to the top of the pillars was made with a micro-probe to effect the selection of different ones of the lasers. Most apparently, a permanent type of electrical lead is required for the tops of the pillars. In most semiconductor fabrication, a surface electrical lead is provided by vapor depositing aluminum or other conducting material between the two points to be connected. The surface topography is usually gentle enough that the lead material can be smoothly deposited over the transition from one vertical level to another. However, Jewell et al achieved their extremely dense laser array by milling precisely vertical walls to form a high aspect-ratio pillar, e.g., a 5 .mu.m high pillar of 2 .mu.m diameter. Depositing lead material on such a pillar presents a formidable challenge.
A related problem arises from the fact that the lasers have only fractional efficiency in converting electrical current to light. Therefore, a substantial amount of heat is generated in the pillars. Thermal conductivity to the air surrounding the pillars is minimal. The high aspect ratios of the pillars prevent effective heat sinking of the pillars to the substrate. For this reason, CW operation of the laser arrays has been more difficult to achieve than pulsed operation.
There have been suggestions to regrow electrically insulating material in the valleys between the pillars. Such regrowth would provide a degree of planarization so that lead deposition becomes feasible and thermal conduction is improved. Substantial regrowth has not as yet been achieved. It will remain difficult to completely planarize the chip. In any case, the regrowth will be difficult and expensive.
Regardless of how the leads are formed on the vertical-cavity, surface-emitting laser array, a further problem arises. Arrays of 1000.times.1000 are feasible with the present technology. Each of the 10.sup.6 lasers can be individually selected if a separate lead is provided to the top of each laser. Such selectivity is desirable for a number of applications. However, 10.sup.6 separate leads are not feasible. A possible application of a large laser array is for a switchable optical bus or interconnect. A possible size is 32.times.32. The individual contacting scheme requires 1024 leads, perhaps attainable but expensive.
The same problem arises in semiconductor memory chips. There, the problem is solved by matrix addressing in which the array of elements are arranged in rows and columns. One set of conductors are laid in the row direction and a second set in the column direction. The elements in the array are connected between their respective row conductors and column conductors. To select a particular element, only the row conductor and the column conductor associated with that element are selected. Thereby, only 2N leads are needed for a N.times.N array. Adapting the Jewell et al array to the usual types of matrix addressing presents many difficulties.